1. Field of the Invention
The invention relates to design of a continuously tunable inductor applied to an inductance-capacitance (LC) tank voltage controlled oscillator (VCO).
2. Description of Related Art
High performance clock generators are often based on LC VCOs because the high quality factor or Q-factor of the LC tank allows achieving good phase noise performance and low jitter generation. The quality factor is a measure of how efficient the ratio of stored energy to power losses is exploited in an oscillating system. For electrical oscillators it can be expressed as
  Q  =                  ⁢              (        Z        )                          ⁢              (        Z        )            where Z denotes the impedance of the resonating structure, e.g. the LC-tank or one of its components.
The LC-tank of state-of-the-art LC VCOs typically consists of an inductor with a fixed inductance value that is connected in parallel to a set of varactor banks for frequency band selection or coarse tuning and an additional fine tuning varactor. If only a relatively small frequency band of typically less than 15% has to be covered and the mid-band frequency is not extremely high, like, for example, <15 GHz, such a state-of-the-art LC-tank design with a fixed value inductor might be sufficient. The situation changes if either a wide tuning range, for example, one octave, or a very high frequency range has to be covered and additionally, for cost saving reasons, only a cheap process technology like a digital CMOS process is available that does not offer varactors with a high Q-factor.
A wide frequency tuning range would be desired for many varactor banks that degrade the Cmax/Cmin-ratio of the fine tuning varactor due to their fixed value capacitive parasitics. The high number of varactor banks might also degrade the overall quality factor of the LC-tank because of the additional resistive losses that occur with the long wiring connections from the different varactor banks to the inductor of the LC-tank.
If a very high frequency range needs to be covered, the quality factor of the varactor becomes smaller than the Q-factor of the inductor. This is caused by the fact that the reactive part of the varactor impedance is inverse proportional to the frequency (Zvar˜1/jωC) while the inductor impedance increases with increasing frequency (Zind˜jωL).
In either of the above mentioned cases it might be beneficial if a continuously tunable inductor would be available in order to save some of the varactor banks or even to replace the continuously tunable fine tuning varactor by a continuously tunable inductor.